1. Field of the Invention
The present invention relates to a CMOS (Complementary Metal Oxide Silicon) image sensor, and more particularly, to a CMOS image sensor and a method for detecting color sensitivity of red, green and blue light without using a color filter layer.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor can be broadly categorized into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor.
The charge coupled device (CCD) includes a plurality of photodiodes (PD) aligned in matrix-type configuration and converting light signals into electric signals, a plurality of vertical charge coupled devices (VCCD) formed between each vertical photodiode aligned in a matrix-type configuration and vertically transmitting electric charges generated from each photodiode, a horizontal charge coupled device (HCCD) horizontally transmitting the electric charges transmitted by each of the vertical charge coupled devices (VCCD), and a sense amplifier sensing and outputting the horizontally transmitted electric charges.
However, the charge coupled device (CCD) has disadvantages of a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photo process. In the charge coupled device (CCD), a control circuit, a signal processing circuit, an analog to digital (A/D) converter circuit, and so on cannot be easily integrated into a charge coupled device chip, thereby having the problem of forming compact-size products.
Recently, the complementary metal oxide semiconductor (CMOS) image sensor has been considered to be the next generation image sensor that can resolve the problems and disadvantages of the charge coupled device (CCD). The CMOS image sensor is a device adopting a CMOS technology using the control circuit, the signal processing circuit, and so on as a peripheral circuit, so as to form MOS transistors corresponding to the number of unit pixels on a semiconductor substrate, in order to sequentially detect the electric signals of each unit pixel by using a switching method, thereby representing an image.
Since the CMOS image sensor uses a CMOS fabrication technology, the CMOS image sensor is advantageous in that it has low power consumption and has a simple fabrication method through less photo process steps. In the CMOS image sensor, a control circuit, a signal processing circuit, an A/D converter circuit, and so on can be integrated in a CMOS image sensor chip, thereby enabling the product to be fabricated in a compact size. Accordingly, the CMOS image sensor is currently being extensively used in various applied technologies, such as digital still cameras and digital video cameras.
A general CMOS image sensor will now be described with reference to the accompanying drawings.
FIG. 1 is an equivalent circuit diagram of one unit pixel in a general CMOS image sensor. FIG. 2 is a cross-sectional view of a related art CMOS image sensor.
Referring to FIG. 1, a unit pixel of a general CMOS image sensor is formed of one photodiode (PD) and three nMOS transistors (T1, T2, and T3). A cathode of the photodiode (PD) is integrated to a drain of the first nMOS transistor (T1) and a gate of the second nMOS transistor (T2). Also, sources of both first and second nMOS transistors (T1 and T2) are connected to a power line supplying a reference voltage (VR). A gate of the first nMOS transistor (T1) is connected to a reset line providing a reset signal (RST). Then, a source of the third nMOS transistor (T3) is connected to a drain of the second nMOS transistor (T2), the drain of the third nMOS transistor (T3) is connected to a reading circuit (not shown) through a signal line, and the gate of the third nMOS transistor (T3) is connected to a column select line providing a select signal (SLCT). Therefore, the first nMOS transistor (T1) is referred to as a reset transistor, the second nMOS transistor (T2) is referred to as a driving transistor, and the third nMOS transistor (T3) is referred to as a selecting transistor.
The structure of the CMOS image sensor will now be described in detail.
Referring to FIG. 2 illustrating a photodiode region, an active region and a field region are defined on a p-type semiconductor substrate 10, and an isolation barrier 11 is formed in the field region. Then, n-type impurity ions are implanted into areas for photodiodes of the active region, thereby forming photodiodes PD 13a, 13b and 13c for a photodiode array 13.
After that, a transparent insulating interlayer 15 is formed on an entire surface of the p-type semiconductor substrate 10 including the photodiode array 13, and color filter layers of red, green and blue 17a, 17b and 17c for a color filter array 17 are formed on the transparent insulating interlayer 15 corresponding to the photodiodes 13a, 13b and 13c. Also, a planarization layer 19 is formed on the transparent insulating interlayer 15 including the color filter layers 17a, 17b and 17c. 
Then, micro lens 21 are formed on the planarization layer 19 corresponding to the color filter layers 17a, 17b and 17c to focus light thereto. At this time, the semiconductor substrate 10 is formed of a p-type single crystal silicon substrate, and the photodiodes 13a, 13b and 13c are formed of n-type diffusion regions. The photodiode array 13 may be replaced with a photo-gate array using a photo gate. Generally, the transparent insulating interlayer 15 is formed of an oxide layer. Also, the color filter layers 17a, 17b and 17c are formed of photosensitive layers using red, green and blue color dyes.
In case of the CMOS image sensor having the above-described structure, it is general to sequentially form the color filter layers 17a, 17b and 17c on the transparent insulating interlayer 15. More specifically, the photosensitive layer of red dye is coated on the insulating interlayer 15 in a spin-coating process, and then exposed with light and developed, so that the photosensitive layer remains in the color filter area of the insulating interlayer 15 positioned in perpendicular to the photodiode 13a, and the photosensitive layer in the other areas is completely removed, thereby forming the color filter layer 17a. In this manner, after forming the color filter layer 17b corresponding to the color filter area of the insulating interlayer 15 positioned in perpendicular to the photodiode 13b, the color filter layer 17c is formed in the color filter area of the insulating interlayer 15 positioned in perpendicular to the photodiode 13c. 
In the related art, the coating, exposing and developing processes should each be repeated three times to form the color filter array 17 having the red, green, and blue color filter layers 17a, 17b and 17c, which not only complicates the fabrication process of the color filter array but also prevents the transmissivity of the red, green, and blue light rays each passing through the color filer layer from being uniformly maintained.
Recently, in order to resolve such problems of the color filter array, many alternative methods of sensing each of the red, green, and blue light rays without using the color filter have been proposed. Among the proposed methods, a method of using a micro prism is disclosed in the Korean Patent Application No. 10-2003-0056096. Also, a method of using a multiple slits is disclosed in the Korean Patent Application No. 10-2002-0039454. However, in the above-referenced methods, the fabrication processes of the micro prism and the multiple slits are very complicated and have many limitations in essentially resolving the problem of the complicated fabrication process of the CMOS image sensor.